Setting: Gink & Go are at the ice cream shop drinking a milk shake talking about their recent biology class about water transport in plants.

Gink:		What did you think about Saupe’s lecture on the mechanism of water transport?
Go:		I thought it was cool that water is sucked up out of the plant; just like drinking this milk shake through a straw.
Gink:		In my opinion the only thing that sucks is Saupe’s lecture.
Go:		Oh come on now….it wasn’t that bad. We should review our notes - It will help for the test.
Gink:		(muttering) you always spoil a perfectly good afternoon. Anyway, I thought water was pumped to the tops of plants by a root pump.
Go:		That’s a common misconception. Plants can develop root pressures of up to 0.3 megapascals, which is about 45 pounds per square inch, but this isn’t nearly enough pressure to get water to top of a tall tree.
Gink:		Oh right - Saupe said that at least 3 megapascals was needed to move water to the top of a redwood tree.
Go:		In addition, a pump needs living cells and Eduard Strasberger showed long ago that water is transported even after stems are treated with cell poisons.
Gink:		Big deal. So water must move to the leaves by climbing up the thin tubes of xylem cells.
Go:		Do you mean capillary action?
Gink:		Duh, right brainiac!
Go:		That’s a good hypothesis because the height a water column moves up a tube is inversely related to the diameter of the tube.
Gink:		So water moves higher up a skinny tube than a thick one.
Go:		That's right and we can calculate how high it moves in meters by dividing the number, 14.87, by the tube radius in micrometers.
Gink:		And since xylem cells are so narrow I'll bet water can be transported to the tippy top of any tree, even a stupid redwood.
Go:		Not exactly. If we substitute the diameter of` a typical vessel in the equation, which is about 10 �m, then water could only move by capillarity to about 0.15 meters.
Gink:		Hmmm....That’s a little short. So, you’re telling me that water is sucked up to the leaves?
Go:		You got it!
Gink:		But, what are the lips? What’s doing the sucking?
Go:		Air
Gink:		Huh....?
Go:		Well, more specially, the water potential gradient between the leaf and the air. The water in the leaf has a relatively high water potential, about -1 MPa but air has a very negative water potential, about -100 MPa. This difference is the driving force for the tendency of water to evaporate from the leaf.
Gink:		As one molecule of water evaporates from the leaf I suppose that it pulls the another one behind it all the way down the column of water to the root.
Go:		You’ve got it. And it only works because of the cohesive and adhesive properties of water.
Gink:		It sounds a little like jumpers parachuting from a plane while holding hands.
Go:		That’s an excellent analogy.
Gink:		But, if this model is correct, then the water transport tubes can’t have much resistance.
Go:		You’re right – they don’t! The vessels and tracheids are hollow. At maturity the cells die and he contents are reabsorbed which makes a continuous column of water from the leaves to the soil
Gink:		But if the water is being pulled up, then is the xylem under a suction?
Go:		It is. This negative pressure is also called a tension.
Gink:		But how do you know that there is a tension in the xylem?
Go:		A couple of ways. First, you can measure the diameter of the tree with a dendrometer. It shows that diameter of the stem is wider at night than the daytime.
Gink:		This is just like putting your finger on the end of straw and sucking; the straw gets thinner.
Go:		Right. In addition, if you puncture the xylem with a knife, it will suck in fluids, which means the pressure inside the stem is lower than outside.
Gink:		Oh yeah just like that goofy video clip Saupe showed us. But won't the water columns be under so much tension that they break?
Go:		It's true that they are under significant stress, but some elegant experiments in which z-shaped tubes filled with water were centrifuged showed that the water could withstand very large forces and would hold together by cohesion
Gink:		So why don’t the xylem cells implode if they are under so much tension?
Go:		That’s because they have thickenings in the wall to strengthen them
Gink:		My brain is getting weak from this geek talk. Let’s finish sucking our malts and go watch some TV.